ESO help fails for omegacam

The ESO help tool fails to parse the [OMEGACAM page](http://archive.eso.org/wdb/wdb/eso/omegacam/form):

```
>>> Eso.query_instrument('omegacam', help=True)
INFO: List of the column_filters parameters accepted by the omegacam instrument query. [astroquery.eso.core]
INFO: The presence of a column in the result table can be controlled if prefixed with a [ ] checkbox. [astroquery.eso.core]
INFO: The default columns in the result table are shown as already ticked: [x]. [astroquery.eso.core]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/astroquery/eso/core.py", line 329, in query_instrument
    self._print_help(url, instrument)
  File "/Library/Python/2.7/site-packages/astroquery/eso/core.py", line 642, in _print_help
    form = doc.select("html > body > form > pre")[0]
IndexError: list index out of range
```

Somehow, `doc.select('html > body > form')` yields empty, though `doc.select('html > body')` and `doc.select('form')` both return useful results.  The form is clearly there and win the body, so I don't understand the problem yet.  @jwoillez ?